INVESTIGATION OF THE SURFACE PLASMON-POLARITONS EXCITATION EFFICIENCY ON ALUMINUM GRATINGS, TAKING INTO ACCOUNT DIFFRACTED RADIATION

Detailed studies of the efficiency of excitation of surface plasmon-polaritons (SPP) on aluminum gratings with a period a = 694 nm, which exceeds the incident wavelength of λ = 632,8 nm, have been carried out. The gratings relief depth (h) range was 6–135 nm. Research samples were formed on As40S30Se30 chalcogenide photoresist films using interference lithography and vacuum thermal deposition of an opaque aluminum layer about 80 nm thick. An atomic force microscope was used to determine the groove profile shape and the grating relief depth. The study of the SPP excitation features was carried out on a stand mounted on the basis of a G5M goniometer and an FS-5 Fedorov stage by measuring the angular dependences of the intensity of specularly reflected and diffracted p-polarized radiation of He-Ne laser. When determining the SPP excitation efficiency, the resonance values of both specular reflection and reflection in the -1st DO were taken into account. It was found that the dependence of the integral plasmon absorption on the grating modulation depth (h/a) is described by a somewhat asymmetric curve with a wide maximum, the position of which corresponds to an h/a value of about 0.07 and a half-width of about 0.123. This allows to excite SPP with an efficiency ≥ 80% of the maximum value on the gratings with the 0,05-0,105 h/a range. The half-width of the plasmon minimum of the reflection in the -1st DO is less than in the specular reflection, which can increase sensitivity of sensor devices when registering the shift of the minimum from angular measurements. The dependence of the half-width of the SPP reflection minima on the grating modulation depth is close to quadratic. In the investigated h/a range (from 0.009 to 0.194), the maximum dynamic range of the reflection coefficient is two orders of magnitude and is achieved in specular reflection for gratings with h/a ≈ 0.075.

Excitation of Ultrashort Spin Waves via Spin-Cherenkov Effect in Magnetic Waveguides

The excitation of ultrashort wavelength spin waves via the spin-Cherenkov effect in magnetic waveguides is investigated via a micromagnetic modeling. The proposed excitation method is relatively simple and easily tunable. The excitation efficiency of the proposed scheme is obtained for different excitation pulse velocities and widths. A coupled waveguide system is also considered. In this case, the spin waves are excited in the first waveguide and then are transferred to the second one due to the dipolar coupling between waveguides. It is also shown that the excitation and transfer of excited spin waves have some limitations related to the dipolar coupling mechanism between the waveguides.

Simultaneous Two-Photon Fluorescence Microscopy of NADH and FAD Using Pixel-to-Pixel Wavelength-Switching

Two-photon fluorescence (TPF) microscopy of intrinsic fluorophores provides physiological and pathological information from biological tissues. Reduced nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FAD) are two endogenous fluorescent coenzymes existing on the intracellular scale. Autofluorescence images of NADH and FAD have been applied to noninvasively record changes during metabolism, according to their distributions and concentrations. However, the widely used sequential (non-simultaneous) excitation scheme results in artifacts caused by sample motion or laser power fluctuation. The single-wavelength illumination scheme suffers from low excitation efficiency and spectral bleed-through. In this paper, we demonstrate a new imaging system simultaneously capturing autofluorescence images from NADH and FAD, with high excitation efficiency and negligible spectral bleed-through. Two temporally multiplexed and spatially overlapped excitation beams were achieved with fast-switching light paths based on an electro-optic modulator. The switching beams were centered at 750 and 860 nm, enabling independent excitations of NADH and FAD. Autofluorescence images of NADH and FAD were acquired at the wavelength ranges of 415–455 nm and 500–550 nm, respectively. The electro-optic modulator was synchronized with the pixel clock from the microscope, achieving pixel-to-pixel wavelength-switching. The capability of the system was demonstrated by performing TPF imaging of freshly excised mouse colon tissues. The microenvironment of the colon wall was depicted by the distributions of colonocytes, goblet cells, and crypts of Lieberkühn, and the relative concentrations of NADH and FAD were estimated. The experimental results show that the system can effectively perform simultaneous imaging of NADH and FAD, and is considered a promising tool for investigations into metabolism-associated processes and diseases.

DEPENDENCE OF SURFACE PLASMON POLYARITON EXCITATION EFFICIENCY ON ALUMINUM GRATINGS RELIEF DEPTH

An experimental study of the excitation of surface plasmonpolaritons (SPP) on aluminum diffraction gratings with a fixed period of 519 ± 0,5 nm and a variable modulation depth h/a(where h is the grating depth, and a – its period) was carried out. Gratings with a sine-like profile were formed on vacuum chalcogenide photoresists films by interference lithography and covered with an opaque aluminum film. A Dimension 3000 Scanning Probe Microscope was used to determine the grating groove profile. The characteristics of the SPP were determinedfor28 gratings with h/a ranged from 0,018 to 0,20, by measuring the dependences of specular reflection of p-polarized radiation of He-Ne laser on the angle of incidence, which was defined as the angle between the normal to the substrate plane and the laser beam. It was found that there is an optimal grating relief depth for a given excitation wavelength, which provides the maximum transfer of the incident electromagnetic wave energy to the surface plasmon-polariton mode.The dependence of the SPP excitation efficiency on the grating modulation depth has a maximum at a relatively small value of h/a ≈ 0.086. At such modulation depth the absorption of electromagnetic radiation of the incident laser beam is more than two orders of magnitude higher than the absorption of aluminum film with flat surface at the same angle of incidence. The position of the angle of resonant excitation of SPP practically does not change from h/a= 0,018 up to h/a ≈ 0,06. With further increase of h/a it begins to shift to the region of smaller incidence angles, with the rate of the shift accelerating gradually. With an increase of h/a, a decrease in the depth of the plasmon resonance and a significant increase in its half-width are also observed, and the dependence of the half-width of the SPP band on the modulation depth is close to quadratic. Using this grating-coupled SPP technique, the estimated thickness of air-formed oxide layer on the aluminum gratings surface (about 3.9 nm) is close to the value obtained in the literature with a set of complicated techniques.

Photothermal excitation efficiency enhancement of cantilevers by electron beam deposition of amorphous carbon thin films

In recent years, the atomic force microscope has proven to be a powerful tool for studying biological systems, mainly for its capability to measure in liquids with nanoscale resolution. Measuring tissues, cells or proteins in their physiological conditions gives us access to valuable information about their real ‘in vivo’ structure, dynamics and functionality which could then fuel disruptive medical and biological applications. The main problem faced by the atomic force microscope when working in liquid environments is the difficulty to generate clear cantilever resonance spectra, essential for stable operation and for high resolution imaging. Photothermal actuation overcomes this problem, as it generates clear resonance spectra free from spurious peaks. However, relatively high laser powers are required to achieve the desired cantilever oscillation amplitude, which could potentially damage biological samples. In this study, we demonstrate that the photothermal excitation efficiency can be enhanced by coating the cantilever with a thin amorphous carbon layer to increase the heat absorption from the laser, reducing the required excitation laser power and minimizing the damage to biological samples.

Increasing the Q-factor of thin planar dielectric resonator with whispering gallery modes

The spectral and energy characteristics of the tunable thin planar dielectric resonator with the movable metal mirror are researched. It is found that the high-Q HE-polarized whispering gallery modes (WGMs) are effectively excited in such a resonator by the dielectric waveguide. A mode set of the tunable thin planar dielectric resonator depends on the presence of an air gap in its design. Changing the height of the air gap affects the energy characteristics of the tunable thin planar dielectric resonators (DR). Shifting the resonant fields of WGMs from the dielectric disk to the air gap is the reason for this effect. It is shown that at certain heights of the air gap, increasing the unloaded Q-factor of the tunable thin planar DR and improving the excitation efficiency of WGMs in it is achieved.